Microelectromechanical systems (MEMS) devices are used for a variety of applications including optical switches and displays, microrelays, accelerometers, gyroscopes, image correctors, ink jet printheads, flow sensors, and medical devices. MEMS are fabricated in a fashion similar to microelectronics in the integrated circuit (IC) industry using surface micromachining techniques. Freestanding MEMS structures, such as pivoting mirrors, beamsplitters, lenses, gears, cantilevered beams, and motors, etc. are created at the end of the process flow by removing the oxide matrix surrounding thin film structural members. Polycrystalline silicon (i.e., polysilicon) is to date the most successful MEMS material because many requirements can be satisfied simultaneously. Other structural materials are in use or being explored, such as: aluminum, silicon carbide and “amorphous diamond”.
Surface micromachining, LIGA techniques, and thin film techniques such as chemical vapor deposition, sputtering, or pulsed laser ablation can be used to form MEMS structures. For volume production, the same MEMS device will be repeatedly fabricated over the surface of a large diameter (4–12 inches) silicon wafer. Typically, there are fifty or more identical die sites. The microstructure of the resulting films and structures can exhibit cross-wafer non-uniformities, resulting in variations of thickness, height, residual stress, stress gradient, or elastic modulus across the wafer. Both mechanical and surface properties must be sufficiently well controlled to guarantee that the intended design function of the MEMS device is met across the entire wafer. For example, the resonant frequency of an electrostatic comb drive can be sensitive to small variations in residual stress. Also, highly curved comb drive fingers or suspensions (caused by stress gradient) will result in device malfunction. Furthermore, surface properties such as adhesion and friction are very sensitive to processing, and may exhibit cross-wafer non-uniformity as well. Poor quality control of surface properties may result in failure of devices that rely on contact or sliding of surfaces.
A need exists, therefore, for rapid and accurate, non-contact, three-dimensional imaging and metrology of complex features of MEMS structures (as well as other structures, such as thin film structures, e.g., nanoindentation, microfluidic channels, and biological specimens). One conventional metrology technique is Scanning Electron Microscopy (SEM). However, because of electron charging and calibration problems, it is difficult to obtain the required nanometer scale resolution by this technique. Other metrology techniques, such as AFM (Atomic Force Microscope) and contact profilometry, can provide the required nanometer-scale resolution to accurately measure 3-D out-of-plane features of IC's and MEMS devices, but either require extensive sample preparation, or rely on potentially destructive contact with the sample surface. Other non-contact techniques, such as conventional light microscopy, do not provide the required nanometer-scale resolution.
In U.S. Pat. No. 5,990,473, Dickey and Holswade describe an apparatus and method for sensing motion of MEMS structures by reflecting or scattering light off of a corrugated surface (e.g., gear teeth) of a movable member (e.g., a gear). However, this system does not provide nanometer-scale measurement of the surface topography of the MEMS structures.
Optical interference microscopes (e.g., optical profilers) can provide the required accuracy (nanometers to sub-nanometers). These non-contact, non-destructive devices use quantitative interferometric phase-measurement techniques to determine the phase difference between an object point on the sample and a reference surface (typically an optically flat reference mirror). The measured phase difference is then converted into topological information. Computerized analysis of a series of interferograms taken while the reference phase of the interferometer is changed (e.g., by using phase-shifting interferometry) provides rapid and accurate determination of the wavefront phase encoded in the variations of the intensity patterns of the recorded interferograms, requiring only a simple point-by-point calculation to recover the phase. The use of phase-shifting interferometry (PSI) conveniently eliminates the need for finding and following fringe centers. PSI is also less sensitive to spatial variations in intensity, detector sensitivity, and fixed pattern noise. Using calibrated PSI, or similar computer analysis techniques, measurement accuracies as well as 0.1 nanometers can be attained if there are no spurious reflections from interfaces other than the one of interest.
It is highly desirable to perform metrology of IC's and MEMS devices at the wafer scale using a microscope setup on a conventional microelectronics probe station that can align wafers and move rapidly from one die site to the next. During electrical probing of a wafer on the probe station, released MEMS structures can be electrically activated; hence, their motion or mechanical behavior can be tested at the wafer scale (e.g., before the wafer is sliced into individual dies). Consequently, a need also exists for measuring out-of-plane deflections, oscillations, or other dynamic 3-D parameters of actuated MEMS devices with high accuracy and low cost. Electrical probing of the wafer requires a long working distance between the end of the microscope (e.g., tip of the sample objective) and the face of the wafer to permit access from the side of the wafer by a standard commercial electrical probe arm or probe card. The required working distance can be as large as 20–30 mm, depending on the number and size of probes needed to simultaneously reach across the wafer from the side.
Commercially available interference microscopes (e.g., the New View 5000 3-D Surface Profiler manufactured by Zygo, Inc., Middlefield, Conn., or the NT2000 3D Optical Profiler manufactured by Wyko, Inc. of Tuscan, Ariz.) do not have the necessary long working distance required for imaging MEMS structures while being actively probed. Typically, commercial interference microscopes have a free working distance less than approximately 10 mm. This is because they use a special interferometer attachment (e.g., Mirau, Fizeau, or Michelson interference attachment), which contains a beamsplitter and reference mirror surface in a compact arrangement. The interferometer attachment is commonly located in-between the standard sample objective and the sample's surface. This arrangement unfortunately reduces the available free working distance to less than 10 mm (especially at higher magnifications, e.g., 20–50×). Additionally, in this configuration interference fringes cannot be easily obtained through a transparent window (such as might be found in a vacuum chamber) due to the phase shift induced by the window. A need exists, therefore, for an interferometric microscope that has a long working distance, and that can easily image through a transparent window.
A Linnik-type interference microscope (i.e., microinterferometer) provides a long working distance and allows the use of high magnification objectives having high numerical apertures. See U.S. Pat. No. 4,869,593 to Biegen; also U.S. Pat. No. 5,956,141 to Hayashi; also Advanced Light Microscopy, Vol. 3, by Maksymilian Pluta, Elsevier Science Publishers, Amsterdam, 1993, pp. 334–347.
FIG. 1 (prior art) illustrates a schematic layout of a standard Linnik microinterferometer, which is based on a Michelson-type two-beam interferometer. For proper operation of this interferometer, the sample objective and the reference objective must be as close to identical optically as possible. Normal manufacturing tolerances do not guarantee that any two objectives intended to be the same (i.e. having the same design and manufacturing specifications) are sufficiently identical for use in a Linnik interferometer. For this reason, optically identical objective pairs for use in a Linnik interferometer are obtained by sorting through a larger number of objectives to find a pair that is sufficiently identical. We will refer to such a pair of well-matched objectives as being “optically identical.” In a Linnik microscope, the illumination beam is split into two beams by means of a beamsplitter. The reference beam in the reference arm is directed onto and reflects off of a reference surface (i.e., the reference mirror). The object beam (i.e., sample beam) in the sample arm (i.e., sample circuit) impinges onto and then reflects off of the sample's surface (e.g., MEMS device). The two beams are then recombined after passing back through the beamsplitter, thereby forming an interferometric image (i.e., interferogram) of the sample's surface at the image plane of the microscope.
Most commercially available interference microscopes utilize an incoherent source of light, which limits the coherence length to less than 50 μm. With such a short coherence length, the optical path lengths of the reference arm and the object/sample arm must not differ by more than approximately ˜1 μm in order to achieve high contrast interference fringes. An additional requirement is that straight interference fringes be obtained when viewing a sample having an optically flat surface. This requirement is only satisfied when the wavefront curvature of the reference beam precisely matches that of the sample beam. When using incoherent light, these two requirements imply that optical path lengths of the sample arm and the reference arm must be precisely matched, and that the distance from the beamsplitter to the back focal planes of the sample and reference objective assemblies must also be precisely matched.
As described previously, these requirements are satisfied in a standard Linnik interferometric microscope by sorting through and optically testing a large batch of objectives and selecting a pair of “optically identical” objectives. As illustrated in FIG. 1, the pair of optically identical objectives is used to produce high-contrast interference fringes with minimum curvature. It is difficult, time-consuming, and, hence, expensive to obtain two optically identical long working distance (LWD) sample objectives, especially at high magnification (e.g., 50×). In addition, changing the overall magnification of the microscope (e.g., from 5× to 20×) requires that both of the optically identical objectives be changed at the same time, and replaced with another pair of (different power) optically identical objectives, which adds additional time and expensive. So, having a system with, for example, four different magnifications (e.g., 5×, 10×, 20×, and 50×) would require four pairs of optically identical objectives, for a total of eight objectives, which becomes quite expensive.
For any interference microscope, it is important that the system produces high quality interference fringes. In a classic Linnik interference microscope the use of coherent (e.g., laser) illumination can alleviate the problems associated with mismatched optical path lengths because of the long coherence lengths characteristic of laser light. When laser light is used, high quality interferograms can be obtained even when the reference and sample arms have substantially different optical path lengths. This approach is described in U.S. Pat. No. 6,721,094 to Sinclair et al., which is herein incorporated by reference.
However, the present invention uses incoherent illumination, which has several advantages over coherent (e.g., laser) illumination, including:                1. Incoherent illumination provides high image quality due to the lack of speckle that is commonly associated with coherent (laser) light sources.        2. Incoherent illumination provides the ability to distinguish fringes due to the finite fringe envelope characteristic of incoherent illumination; which allows topographic information to be obtained even in the presence of discontinuous surface features.        3. With coherent illumination, all fringes appear identical and there is no simple method to determine the magnitude of a surface step.        4. Incoherent illumination provides the ability to reject reflections from surfaces not under investigation due to the small temporal coherence. Conversely, with coherent illumination, the partial reflections from any interface in the illumination path will combine with the reflection from the surface of interest and produce unwanted interference fringes.        5. Incoherent illumination allows lower cost and compactness of a LED (incoherent) source, as compared to a laser. Multiple illumination wavelengths may easily be employed.        6. Incoherent illumination allows easy implementation of stroboscopic illumination for characterization of repetitive events by simply strobing the bias current to the LED.        7. Incoherent illumination provides the ability to perform real-time transient measurements (i.e., not stroboscopic) without speckle artifacts that accompany coherent illumination (even with using a rotating diffuser).        
Gale, et al. describe a Linnik microscope capable of illumination by one of two different sources, namely, a tungsten halogen lamp or a helium-neon laser (see D. M. Gale, M. I. Pether, and J. C. Dainty, “Linnik Microscope Imaging of Integrated Circuit Structures”, Applied Optics Vol. 35, No. 1, January, 1996, pp. 131–137). However, this system requires a pair of optically identical objectives (with an aberration less than one-eighth of the operating wavelength), because this is required when the incoherent tungsten halogen lamp is used (i.e., due to the requirement for matching both the optical path lengths and wavefront curvature when using incoherent light).
A need remains, therefore, for a long working distance, incoherent interference microscope that produces flat, high quality, high contrast fringes, and that does not require the use of a expensive, optically identical pairs of sample and reference arm objectives. A need exists, also, for an interference microscope that uses an inexpensive reference beam circuit that requires only minor adjustments when changing sample objections to obtain different magnifications. There is also a need to allow imaging through a glass window that requires only minor adjustments of the system.
Against this background, the present invention was developed.